1. Field of the Invention
This invention relates generally to fault-location systems for determining the distance to a fault point and more particularly, it relates to a fault distance locator for underground cable circuits and a method for the same for calculating more accurately the location of a cable fault from a source-connected monitoring location in an efficient and effective manner.
2. Description of the Prior Art
As is generally known to those in the electric utility industry, buried underground cables utilized in the United States are typically formed with a center conductor surrounded by an outer polymeric insulation and a concentric neutral disposed over the polymeric insulation. These underground cables are employed to serve for the distribution or transmission of electrical voltage in the medium range between 15 KV and 35 KV. Faults sometimes develop, such as when the cable is punctured creating a short circuit between the conductor and the concentric neutral, which require the repair or replacement of the cable or a portion thereof. In order to facilitate the correction of the fault, it is desirable to know the exact location of the fault.
To this end, electric utilities have constructed access points which are generally provided at pedestals or towers that are located at spaced apart positions along the underground cables. Typically, the cable lengths are approximately 1 mile or longer with the access points being disposed about 500 feet apart for underground residential distribution (URD) or underground commercial distribution (UCD) circuits. At these various access points, there are provided cable circuit switches and faulted circuit indicators (FCI) which are located inside a transformer and switchgear box.
When a cable failure occurs, a fuse or circuit breaker or other circuit interrupting or protective device will be tripped so as to cause a circuit interruption. A linecrew will be sent to inspect the FCI in the transformer and switchgear box at each switch location to determine the last FCI unit (tripped) to indicate the passage of a fault current. In this manner, the faulted cable section can be located. With this information, the fault cable section can then be "switched out" or isolated so as to become a new open-loop point. Consequently, full restoration of service is provided during the time when the faulted cable section is being repaired. This procedure just described is commonly followed by virtually all of the electric utilities in the United States and involves about a 2 to 4 hour period of time to be accomplished.
This approach has several major disadvantages. For example, each of the transformer and switchgear boxes must be located, which may be covered or hidden by shrubs, bushes or other debris, and the locked doors thereof must be opened (which may be rusted) in order to check the status of the FCI units therein. This is a very time-consuming and laborious task. Further, prior to the fault locating step the cable is to be de-energized, tested for potential, and grounded so as to remove the charge due to the cable capacitance. Thus, this method requires additional time and labor expense which is a slow and tedious process.
After the steps of determining of an existence of a cable fault and determining the approximate location of such fault, linemen or repairmen later return in several days to find the exact and actual location of the fault so that the appropriate repairs can be made in order to restore the circuits to their previous normal operation.
The various procedures to be followed after the cable is de-energized and the different cable fault locating techniques are outlined in an article entitled "IEEE Guide for Fault Locating on Unshielded Power Cable Systems" and drafted by the Insulated Conductors Committee Draft 6 dated Dec. 17, 1993. The variety of techniques described in this article included time domain reflectrometry, arc reflection, surge pulse, capacitive discharge (thumper), and fault reduction (burning). However, all of these methods require the use of costly special equipment in order to determine the distance to the fault or otherwise pinpoint the fault location with reasonable accuracy. For example, there is a commercially available prior art reflection system using a memory radar which is manufactured by VON Corporation of Birmingham, Ala. This prior art system is typically quite heavy and is mounted on a special van or truck and must be operated by specially-trained crewmen on site after the testing has been performed on the de-energized cable. Such a system may cost as much as $10,000.00 and other similar commercialized equipment may run up to $100,000.00.
There is also known in the prior art of a digital fault locator which calculates the reactance of a faulty line, with a microprocessor, using the one-terminal voltage and current data of the transmission line. This digital fault locator has been described in an article entitled "Development of a New Type Fault Locator Using the One-Terminal Voltage and Current Data" by M. Yamaura et al., published in IEEE Transactions on Power Apparatus and Systems, Vol. PAS-101, No. 8, August 1982, pp. 2892-2898. This paper also stated generally that one of the methods for measuring the distance to a fault point on a transmission line is to set off pulses when a fault occurs and the pulse return time from the fault point is utilized to determine the location of the fault point. This same comment is mentioned in U.S. Pat. No. 4,313,169 to T. Takagi et al. issued on Jan. 26, 1982, at column 1, lines 18-26. However, the inventors of the present invention are unaware of any such fault locators in commercial use which rely upon this principle.
There also exists a number of prior art patents that have been granted which are directed to fault locators of the type for determining the location of high resistance ground faults by using impedance based calculations. In other words, the voltage and the current are measured at both ends of the faulted line and some algorithm would make the calculation of the distance to the fault based upon the measurements. However, none of the prior art fault locators considered the effect the arc voltage had on the calculations, which could be very high, in the case of overhead lines, thereby preventing the pinpointing of the fault location with a high degree of accuracy. The prior art patents are believed to be best exemplified by U.S. Pat. Nos. 3,474,333; 4,107,778; 4,314,199; 4,559,491; 4,785,249; 4,857,854; 4,868,507; 4,909,937; and 5,160,926.
For example, in U.S. Pat. No. 3,474,333 to H. Hoel issued on Oct. 21, 1969, there is disclosed an apparatus for determining the distance between a supervisory station in an A.C. system and a fault affecting the system. The apparatus includes means for measuring the voltage when the current traverses the zero datum and means for determining the derivative of the current at that instance. An output device determines from this information the inductive reactance under the fault condition to provide an indication of the distance to the fault.
In U.S. Pat. No. 4,107,778 to Y. Nii et al. issued on Aug. 15, 1978, there is disclosed a digital fault-location calculation system for a large electric power transmission system. There are provided voltage and current transformers 12 and 14 for measuring the respective voltage and current. A process unit 16 is coupled between the transformers and a computer 18 and includes a sampling-A/D converter. The digital computer includes a CPU 20, a memory device 22, and a control electronic circuit 24. The computer functions to detect the occurrence of the fault in the power transmission line and to calculate accurately the distance from an installation point of the transformers to the fault location point in accordance with predetermined processes.
In U.S. Pat. No. 4,559,491 to M. Saha issued on Dec. 17, 1985, there is taught a method for locating a fault point within a section of a three-phase power transmission line located between a network positioned behind the section and a network positioned ahead of the section. Currents and voltages are measured at a measuring point at one end of the section. The distance between the measuring point and the fault point is computed on the basis of the measured values and the parameters of the section.
U.S. Pat. No. 4,857,854 to T. Matsushima issued on Aug. 15, 1989, teaches a digital fault locator for locating a fault point of a power transmission system which includes a memory for storing electric amounts of an input from the power transmission system and a plurality of digital filters having different filter functions to which the electric amounts are supplied from the memory. Specific ones of the outputs of the digital filters are selectively used according to a persisted time of a fault to measure a distance to the fault point.
U.S. Pat. No. 4,868,507 to E. Reed issued on Sep. 19, 1989, shows a resistance fault locator circuit for determining the distance to a fault between two conductors of a cable. The fault locator circuit includes a digitally-controlled current split circuit 29, an error amplifier 9, a programmed computer 37, and relay switches for establishing several modes of operation of the circuit.
U.S. Pat. No. 4,906,937 to K. Wikstrom et al. issued on Mar. 6, 1990, shows a method and device for determining the location of a fault on a power transmission line between two stations. The phase voltage and the phase current or changes in the phase current are measured on the occurrence of a fault. These measured values are low pass filtered and then converted from an analog to an instantaneous digitized current and voltage values. The fault distance and an apparent fault resistance at the fault location are calculated and then compared with the respective upper and lower limits. A tripping signal is generated by a logic unit when the fault position and the apparent fault resistance are within the limit values, respectively.
Aside from these prior art patents, there are also known heretofore the use of microprocessor-based relays by electric utilities for protecting transmission systems against overcurrents and for locating faults. Further, the use of microprocessor technology relative to protective relay systems is described in an article by Stanley E. Zocholl entitled "Integrated Metering and Protective Relay Systems," published in IEEE Transaction Industry Applications, Vol. 25, No. 5, Sept./Oct. 1989, pp. 889-893.
There still exists another prior art system which utilizes a single-ended impedance-based monitoring of the fault parameters to calculate the distance to a fault with the aid of a microprocessor. A report of this work is described in the paper entitled "An Advanced Technology To Automate The Location of URD Cable Faults," written by E. E. Baker, Paul Pearson, Jr., A. P. Sakis Meliopoulis, and A. C. Westrom and was presented to the Edison Electric Institute T & D Committee on Apr. 7, 1993. This prior art system assumed a fixed arc-voltage in the single-ended impedance-based calculation. It was found from an analysis of the test data collected made on a variety of actual cable faults that the system had an unacceptable degree of accuracy in pinpointing the fault location. This was caused by very large and continuously variable arc voltages experienced at the fault location which could range from a few hundred volts to 3500 volts on a 7.2 KV circuit. It was determined that variability of the arc voltage will typically exist within each half-cycle of arcing. Based on the studies conducted, it was concluded that the single-ended impedance-based system was not practical for cable fault determination due to the inability to monitor the effect of the arc voltage at a location remote from the fault point.
In view of the electric utilities increasingly employing more underground installations in recent years, there has been a corresponding increase in the number of cable faults. As a result of this trend in technology, there has been a continued need for an improved fault-locating apparatus which can determine the location as quickly as possible after the inception of a fault and which can pinpoint the location of such fault with a high degree of accuracy.
The present invention represents a significant improvement over all of the aforementioned prior art. Based upon recent test data, the present invention has the capability of determining the fault location within five feet or less of the faulted cable location. This is accomplished in the present invention by the use of a pulse generator for injecting a series of chirped pulse streams into the cable shortly after the occurrence of the cable fault. The delay times between the pulses sent on the faulted cable and the reflected pulse signals are used to calculate the distance to the cable fault.